In the communications cable industry, it is well known that changes in ambient conditions lead to differences in water vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High levels of condensed moisture inside a cable sheath system may have a detrimental effect on the transmission characteristics of communications cables whether the particular transmission media is metallic conductors or optical fibers. The presence of moisture is particularly destructive when the cable will be exposed to colder environments where the moisture may freeze and introduce physical stresses and strains on the fiber. Furthermore, moisture may enter the cable because of damage to the cable if the integrity of the cable's sheath system is compromised. For example, rodent and/or termite attacks, as well as external mechanical impacts or forces may cause openings in the sheath system of the cable to occur, allowing water to enter, and, if not controlled, to move longitudinally along the cable into splice closures. Consequently, it should be no surprise that cables for transmitting communications signals must meet industry standards with respect to waterblocking provisions. For example, one industry standard requires that there be no transmission of water under a pressure head of one meter in one hour through a one meter length of cable.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and along the core. For example, sometimes a metallic shield is used to protect a telecommunications cable against lightning, rodent and/or termite attacks. However, the forming of such a shield about a cable core requires the use of relatively low manufacturing line speeds. Also, the use of a metallic shield negates certain benefits resulting from what may otherwise be an all-dielectric optical fiber cable. Further in this regard, any lightning strikes attracted to the metal, as well as gradual corrosion, may cause holes in a metallic shield.
It is not uncommon to include provisions in addition to or as an alternative to a metallic shield for preventing the ingress of water into the core. Filling materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the longitudinal movement of any water which enters the cable. However, due to their physical make-up, the use of such materials often cause housekeeping problems for field personnel, particularly during splicing operations.
Typically, the compositions of matter used as filling materials, sometimes referred to as waterblocking materials, are semisolid or semiliquid substances that comprise a thickening or gelling agent in a liquid carrier. In optical fiber cables, a further important function of a filling material is the maintenance of the optical fibers in a low stress state as well as the chemical compatibility of the material to the various coating and color layers on the optical fiber itself.
A communications cable filling material, especially an optical fiber cable filling material, should meet a variety of requirements, including industry-standard drip tests. To pass these tests, the physical properties of a cable having such filling materials in its core must remain within acceptable limits over a rather wide temperature range e.g., from about -40.degree. C. to about 80.degree. C. In other words, the filling material should not drip out of cable even at 80.degree. C. Oil separation is a property of a gel-like material which describes the tendency to bleed oil during its lifetime. What is desired is a filling material which has an oil separation no greater than 4% when centrifuged at relative centrifugal forces of 27,000 g at 25.+-.2.degree. C. for two hours.
Further complicating the optical fiber cable situation is that suitable filling materials must yield under strains experienced when the cable is made or handled. Otherwise, movement of the optical fibers within the cable would be prevented and the fibers would buckle because they contact, with a relative small periodicity a surface of the unyielding filling material. Such contact with an unyielding surface unfortunately introduces a large amount of microbending loss in the signal being carrier by that fiber.
In order to adequately address the concerns raised above, filling materials for use in optical fiber cables should have a relatively low shear modulus, G.sub.e. However, it has been determined that, at least for some applications, a low value of G.sub.e of the filling material is not sufficient to assure low cabling loss, and that a further parameter, the critical-yield stress, .sigma..sub.c, may need to be controlled because it also affects the optical performance of fibers in a cable containing common types of filling materials.
One filling material having a relatively low critical-yield stress is disclosed in U.S. Pat. No. 4,701,016 which issued on Oct. 20, 1987 in the names of (C. H. Gartside, III, et al. The disclosed filling material comprises oil, a gelling agent such as colloidal particles, and optionally, a bleed inhibitor. It includes 93% by weight of mineral oil and 7% by weight of hydrophobic fumed silica. Among oils useful in the practice of the invention are ASTM type (ASTM D-226 test) p103, 104A, or 104B, naphthenic oils having a minimum specific gravity of about 0.860 and a maximum pour point (ASTM D97) of less than approximately -4.degree. C., and polybutene oils of minimum specific gravity of about 0.83 and a maximum pour point (ASTM D97) of less than about 18.degree. C. The colloidal particle filler material preferably comprises silica particles. The critical-yield stress of the filling material of the '016 patent is not greater than about 70 Pa (or about 0.01 psi), measured at 20.degree. C., whereas the shear modulus is less than about 13 kPa (or about 1.89 psi), at 20.degree. C.
Another filling material that is attractive for use in optical fibers is described in commonly-assigned U.S. Pat. No. 5,187,763 issued in the name of C. F. Tu on Feb. 16, 1993. However, even though the filling material set forth in the '763 patent adequately addresses some of the technical concerns recited above, it does not provide the necessary properties at elevated temperatures, i.e. about 80.degree. C. In particular, this disclosed filling material does not appear capable of capable of passing EIA/TIA Standard FOTP-81, Compound Flow (Drip) Test for Filled Optical Cable, Section 8 up to 80.degree. C. while maintaining other desired properties such as a critical-yield stress of less than about 0.002 psi.
Because cable drip is related to oil separation, constraints on the sought after filling material include oil separation and critical-yield stress. In addition, the viscosity of the sought-after filling material is important with respect to processing. These constraints usually are antagonistic to each other. For example, a reduction of oil separation and an increase in cable drip temperature require high viscosity and critical-yield stress whereas to facilitate processing and to reduce optical loss requires low viscosity and critical-yield stress.